Cold Air Pools MET
A cold air pool is an area of intense cold air above the surface. It is often caused when a jetstream "snaps off " onto a different path and the cold air is left stranded because of the loss of the impetus of the jetstream to move it along.
Because the air above the surface is so cold, this means a large temperature lapse rate and so very unstable conditions - so air is free to rise, given cumulus or potentially cumulonimbus with the associated precipitation. As with all convective phenomena, this will be worst in mid-afternoon when the temperature at the surface is warmest.
The danger with a cold air pool is that because it is aloft, looking at a surface pressure chart will not alert you to its presence. This is dangerous given the potential hazards it can cause. To find it, you need to look at upper charts for the low pressure it causes.